Method and apparatus for heat forming elongated metal panels

ABSTRACT

A METHOD OF STRETCH FORMING METAL PARTS BY THE PASSAGE OF A HEAT ZONE ALONG A PART HELD IN TENSION AGAINST A DIE SURFACE. THE ENDS OF THE PART OT BE FORMED ARE HELD AND A TENSILE LOAD IS APPLIED HOLDING THE PART IN CONTACT WITH A DIE SURFACE WHILE A MOVABLE HEAT SOURCE TRAVERSES THE PART, LOCALLY YIELDING AID FORMING IT TO THE DIE CONFIGURATION. ELONGATION OF THE PART IS CONTROLLED AS A FUNCTION OF THE   POSITION OF THE HEAT ZONE PERMITTING THE FORMING OF PARTS OF BOTH UNIFORM AND NONUNIFORM CROSS SECTION.

' March 9, 1971 G. A. BOHMANN 3,568,490

METHOD AND APPARATUS FOR HEAT FORMING ELQNGATED METAL PANELS Filed Sept. 27, 1968 JOEPZOU OZCHMI JnEDw mw om 5 Sheets-Sheet 1 INVENTOR GiZRGE A. BOH MANN BY A TORNEYS March 1971 G. A. BOHMANN 3,568,490

METHOD AND APPARATUS FOR HEAT FORMING ELONGATED METAL PANELS Filed Sept. '37, 1968 3 Sheets-Sheet 2 ROOM TEMP.

STRESS (PSI) STRAIN m /in V ELONGATION ACTUAL CURVE THEORETICAL CURVE POSITION OF HEAT SOURCE CONSTANTLY MOVING HEAT SOURCE INVENTOR GEORGE A. BOH MANN ATTORNEYS March 9, 1971 G. A. BOHMANN METHOD-AND APPARATUS FOR HEAT FORMING ELONGATED METAL PANELS Filed Sept. 27, 1968 3 Sheets-Sheet 3 FIG.

ATTORNEYS United States Tatent 6 3,568,490 METHOD AND APPARATUS FOR HEAT FORMING ELONGATED METAL PANELS George A. Bohmann, New Hyde Park, N.Y., asslgnor to Fairchild Hiller Corporation, Farmingdale, N.Y. Filed Sept. 27, 1968, Ser. No. 763,163 Int. Cl. B21d 11/02 US. Cl. 72-302 23 Claims ABSTRACT OF THE DISCLOSURE A method of stretch forming metal parts by the passage of a heat zone along a part held in tension against a die surface. The ends of the part to be formed are held and a tensile load is applied holding the part in contact with a die surface while a movable heat source traverses the part, locally yielding aid forming it to the die configuration. Elongation of the part is controlled as a function of the position of the heat zone permitting the forming of parts of both uniform and nonuniform cross section.

The present invention relates to the stretch forming of large metallic parts such as, for example, the panels which typically are employed to form the skin of an aircraft. It is particularly suitable for those materials, such as titanium, that are generally formed at elevated temperature.

For well known reasons, it is highly desirable that the panels forming the skin of an aircraft have low residual stresses, This requires the application of substantial heat to the panel during or subsequent to the forming process. Generally, prior art techniques involve stretching using a. cold forming die in combination with a large immobile bank of heat lamps covering the entire panel, or a hot die with heat generally supplied by imbedded heating elements. A vacuum forming process is also used in which the panel is vacuum formed into a heated die of the desired contour.

Where the panels to be formed are of excessive lengths, or when the part is non uniform in cross section, the prior art panel forming techniques having proved to be impractical. This is due in part to the power requirements involved for the heat source, the difficulty in evenly heating a panel of extended length to avoid necking when pressure is applied, the mechanical problems of vacuum forming a long panel and the difficulty in compensating for the cumulative buildup of tangential frictional forces between the part and die.

Accordingly, it is an object of the present invention to provide a panel forming process and apparatus which can be employed to fabricate particularly large panels of either uniform or non uniform cross section in a practical and relatively economical fashion and without any post forming heating of the part for stress relieving.

Briefly, in accordance with the invention, pressure is applied to the ends of a panel, forcing the panel against a suitably contoured die. A traveling heat source, adapted to heat a relatively short increment of the panel, is mounted for movement along the length of the panel whereby the panel is incrementally heated as the heat source traverses the length of the panel. The desired elongation of the panel is controlled as a function of the position of the heat source to assure compliance with standard panel forming criteria.

In the drawings:

FIG. 1 is a perspective view of panel forming apparatus according to the invention;

FIG. 2 is a representative stress-strain curve used for explanatory purposes;

FIG. 3 is a graph of total panel elongation vs. heat source position for explanatory purposes;

ice

FIG. 4 is a top view of the device employed to control the position of the heat source; and

FIG. 5 is a sectional view along the line 3-3 of FIG. 4.

In the drawing, an elongated panel 10 (eg. of titanium) is shown applied to a ceramic die 12 which conforms to the desired contour of panel 10. Each end of the panel 10 is inserted between two gripper jaws 14 and 16, respectively and secured within the respective gripper jaws by bolts 18 or hydraulic actuators.

The ceramic die 12 rests on a framework 20 which includes longitudinal I beams 22 and 24. The gripper jaws 16 are secured to the frame 20 at one end, and, at the other end, a pair of hydraulic cylinders 26 and 28 are suitably mounted with respective actuator rods 30 and 32 extending therefrom toward the panel 10. The actuator rods 30 and 32 terminate in clevis 34 and 36 which are bolted to the gripper jaws 14 at 38 and 40, respectively. The hydraulic cylinders 26 and 28 are controlled in the usual fashion by means of fluid control lines 42 and 44, respectively, which can be opened or shut by an electrically controlled valve mechanism 46 shown in diagrammatic form.

A pair of horizontal parallel overhead rails 50 and 52, I-shaped in cross section, are mounted above the panel 10. A travelling heat source 54 rides on the rails 50 and 52 so as to at least traverse the length of the panel 10 supported on the die 12. The heat source 54 includes four vertical posts, 55, 56, 57 and 58, and connecting struts 59a and 59b to which is secured a standard quartz heater 60 or other suitable heating means such as induction coils. Heater 60 may be divided into a plurality (e.g. six) of heater elements 6041-60) which are contoured to the shape of die 12 and which are independently powered by a standard power supply 61.

A roller 62 is mounted at the top of each of the vertical support posts 55, 56, 57 and 58, and adapted to ride on the inner sides of the rails 50 and 52. A variable speed motor 64 is mounted on one end of the rail 50 and includes an output sprocket gear 66 which engages an endless chain 68 supported also by a sprocket 70 extending from a bracket 72 secured to the other end of rail 50. The endless chain 68 is connected at 74 to a lug 76 extending upwardly from the support post 55. Thus, operation of the motor 64 will drive chain 68 thereby causing heat source 54 to move across the length of panel 10. Temperature sensing devices such as radiation pyrometers 77 are mounted on the heat source 54 and connected through feedback loop 79a to the power supply 61 and feedback loop 7% to the drive motor 64. This controls the power input to heat source 54 and the speed of heat source 54 as a function of the heat-up rate of the part 10.

The construction so far described is capable of applying pressure to the panel 10 and simultaneously heating con tinuous increments of the panel along its length. It is also necessary, however, to insure that a desired relationship fexists between the temperature of the panel and the applied orce.

In accordance with standard panel forming techniques, where a panel is to be formed to a desired contour, the force applied should be such that the elastic yield of the material is just exceeded. It is further desirable that the elongation of the panel due to this force be as small as possible. The temperature requirements will vary with the specific material employed. As a typical example, it may be desired to heat a titanium panel to a temperature between 1200 F. and 1350 F., it being desirable to maintain the part at a temperature that will inhibit the introduction of residual stresses during the forming operation. This requirement that the elongation and tem perature of the panel be controlled within certain specified limits requires control of the force applied to the panel and/or the temperature of the panel.

As explained in further detail below, there are a number of different ways this control can be provided. In the preferred embodiment of the invention a simplified construction is used to accomplish this objective, wherein elongation of the panel is controlled as a function of the position of the heat source relative to the panel.

The preferred embodiment operates on the assumption that there is a near linear relationship between the position of heat source 54 relative to panel 10 and the theoretically desired elongation of panel 10 where its yield point is just exceeded. Although, in fact, this relationship is not precisely linear due to temperature and other elfects, the approximation is close enough for use as a practical basis for the procedure. Thus, knowing the temperature to which it is desired to heat the panel 10 during the forming process, it is possible to examine typical stress-strain curves for the material of panel 10 at the desired forming temperature to determine the total elongation which will result if a panel of known length is stressed just beyond its yield point.

FIG. 2 shows typical stress strain curves for titanium 6Al4 v. at room temperature and 1250 F. By examining this curve the strain Ae (measured in inches per inch) can be determined for a sheet which is heated to 1250" F. and stressed into its plastic range just exceeding the yield point. Since the length of the panel is known, the total elongation can be computed by multiplying the panel length (in inches) times Ae. A theoretical curve of elongation versus the position of the heat source can be drawn as shown in FIG. 3. Compensation should be made for the thermal expansion of the panel in the heat affected zone. This can be done by the equation:

where E=actual total elongation L=length of panel I=length of heating zone Atzforming temperature minus ambient temperature k=thermal coefi'icient of expansion t zaverage temperature in that portion of the panel previously traversed by the heat source.

Once the actual curve of FIG. 3 is determined, the movement of the pistons in the hydraulic cylinders 26 and 28 can be adjusted so that they (and thus the panel elongation) bear a desired relationship to the position of the heat source. FIG. 4 illustrates the apparatus by which this control is achieved.

In FIG. 1 an endless chain 80 is secured to a vertical plate 82 suitably attached to the support post 57 for heat source 54. Tension spring 84 assures that the tension in the chain 80 remains constant. The endless chain 80 drives a sprocket 86 and is supported on pulleys 88, 90 and 92, which are mounted in an obvious fashion on the rail 52. A horizontal support bar 94 (FIG. 4) is secured to the frame members 22 and 24, and a base plate 96 is centrally mounted on the support bar 94. Plate 96 includes opposed L-shaped brackets 98 and 100 containing bearing members 102 and 104 in which a lead screw 106 is suitably journalled. The lead screw 106 is directly connected to the sprocket 86 causing it to rotate as the chain 80 and heat source 54 are moved across panel 10.

A retainer member 108 includes a longitudinal threaded aperture 110 (FIG. which engages the lead screw 106 so that the retainer 108 is moved along the axis of lead screw 106 as the lead screw is rotated during movement of the heat source 54. A small displacement transducer 112, which may comprise a standard linear variable differential transformed is suitably secured within the retainer 108. Transducer 112 includes a linearly movable stylus 114 adapted to engage a camming surface 115 of an adjustable cumming bar 116 pivotally mounted at 118 on a plate 120, the voltage output from the transducer being directly proportional to the linear displacement of its stylus 11211. The plate 120 is received between the horizontal flanges of L-shaped brackets 98 and and adapted to slide over the base plate 96. Plate is pinned at 122 to a connecting rod 124, the other end of which is pinned at 126 to the upper gripping jaw 14.

With this construction, the displacement transducer 112 will traverse the camming surface 115 in the direction of arrow 130 (FIG. 4) as the heat source 54 traverses the length of the panel 10. Thus, once the total desired elongation of panel 10 has been determined, the bar 116 can be rotatably adjusted so that the angle of the camming surface 115 (relative to the longitudinal axis of lead screw 106) represents the desired linear function corresponding to the position of heating source 54 relative to the elongation of panel 10. Thus, the output leads of the transducer 112 (shown diagrammatically at 131 in FIG. 1) may be coupled in an obvious fashion to the electrically controlled modulator valve 46 to control the fluid flow in lines 42 and 44 and thus the force applied to panel 10 by hydraulic cylinders 26 and 28 respectively. Modulation control of the fluid flow will assure that the elongation of panel 10 is linearly proportional (in accordance with the predetermined criterion) to the position of heat source 54 relative to panel 10. For example, the amount of fluid flow will vary with the amount that the transducer stylus 114 is depressed. If the stylus 114 is not depressed then the valve will be closed. If the stylus 114 is partly depressed then the valve will be proportionately open. If the stylus is fully depressed the valve will be fully open. This modulation results in panel elongation proportional to the position of heat source 54.

Where a transformer is used as the displacement transducer, additional circuits will be used to provide the required alternating input voltage and to convert the transformer output to direct voltage where required, Such circuits are well known and have been omitted from this description for purposes of simplification. In effect, the arrangement described is an electromechanical comparator which compares actual elongation with a theoretical- 1y desired elongation based upon the actual position of the heat source and adjusts the variables accordingly. It is contemplated that other devices, including all-electrical and tape controlled and programmed comparators, be used for this purpose if desired. I

Basically, there are five variables which affect the operation of the system described. These are the rate of movement of the heat source 54, the tensile load applied to the panel by the hydraulic system, the position of the heat source as a percentage of the total length of travel, the elongation of the panel, and the thermal output of the heat source. The present invention contemplates any possible control of these variables so as to satisfactorily provide incremental panel forming as herein described.

An important advantage of the invention resides in the ability to control the heat profile of the heat source. This renders the invention of particular utility in the forming of panels which are not of uniform (i.e. rectangular) transverse cross section. In such cases the thermal output of different ones of the individual heating zones (lamps or other elements) comprising the heat source can be adjusted (electrically or by mechanical sources) to apply more heat to thicker portions of the panel than is applied to the thinner portions, This assures that the non-uniform part is heated uniformly to the desired temperature despite the different heat-up rates. Where the panel thickness is tapered from end-to-end, the same control may be used if desired and/or the speed of the heat source can be appropriately varied. The invention further contemplates automatic control of the individual heating zones 60a-60f either by a temperature sensing feedback loop or programmed control devices and the like.

A further important feature of the invention includes the addition to heat source 54 of a spray nozzle such as through which a gas such as argon may be applied to cool the panel immediately after the heat source has been applied. This would be a particular advantage for heat treating parts on the die, as for example, the die quenching of titanium 6A14 v. where the part must be quenched within 6 sec. to achieve maximum solution treated properties.

The invention can be used to form parts other than panels, such as extruded and rolled parts. Numerous modifications of the invention will also be obvious to those skilled in the art, and the invention should be defined primarily with reference to the attached claims.

What is claimed is:

1. A method of stretch forming a contoured elongated metallic part, comprising the steps of applying a tensioning force to at least a portion of said part while successively heating longitudinally displaced increments of said portion to a preselected temperature, said tensioning force being greater than the elastic yield point of said part at said temperature.

2. A method according to claim 1, including the step of tensioning said part against a die member, and wherein said step of successively heating increments comprises moving a heat source across said portion.

3. A method according to claim 2, wherein the elongation of the part is caused to bear a predetermined relationship to the position of said heat source.

4. A method according to claim 2, including the step of controlling the amount of heat applied to said heated increment in proportion to the heat-up rate of said heated increment.

5. A method according to claim 2, including the step of controlling the thermal output of said heat source to vary the amount of heat applied to said heated increments.

6. A method according to claim 2, including the step of successively cooling each of said increments after it has been heated.

7. Stretch forming apparatus, comprising a die having a surface corresponding to the desired contour of a part, means for applying a tensioning force to a part applied to said die to cause said part to conform to the shape of said die, means for heating an incremental increment of said part to a preselected temperature, and means for moving said heating means across said part, said tensioning force being greater than the yield point of said part at said temperature.

8. Apparatus according to claim 7, including means for controlling the elongation of the part relative to the position of said heating means.

9. Apparatus according to claim 8, including fluid distributing means for cooling said part after it has been heated by said heating means.

10. Apparatus according to claim 9, wherein said fluid distributing means is secured to said heating means for movement therewith.

11. Apparatus according to claim 8, wherein said means for applying a tensioning force comprises means for gripping said part, said apparatus further including means for comparing the elongation of said part with a desired elongation based upon the position of the heating means relative to the part, and means responsive to said comparing means for moving said gripping means.

12. Apparatus according to claim 8, including means for varying the amount of heat applied by said heating means to said zone.

13. Apparatus according to claim 12, wherein said heating means comprises a plurality of heating elements, and wherein said control means includes means for independently adjusting the thermal output of each of said heating elements.

14. Apparatus according to claim 12 including means responsive to the temperature of said heated increment for adjusting the amount of heat applied to said part by said heating means so as to cause each of said heated increments to attain a preselected temperature regardless of its dimensions.

15. A method according to claim 2, wherein the elongation of the part is caused to bear a predetermined relationship to the position of said heated increment.

16. A method according to claim 15, including the step of controlling the amount of heat applied to said heated increments.

17. A method according to claim 16, wherein the amount of heat is controlled by varying the rate of movement of said heat source.

18. A method according to claim 16, wherein the amount of heat is controlled by varying the temperature of said heat source.

19. A method according to claim 15, wherein different amounts of heat are applied to portions of a heated increment than to other portions thereof.

20. A method according to claim 15, wherein said tensioning force is such as to keep the total elongation of said portion at substantially a minimum value.

21. Apparatus according to claim 14, wherein said means for adjusting the amount of heat comprises means for varying the rate of movement of said heating means.

22. Apparatus according to claim 14, wherein said means for adjusting the amount of heat comprises means for varying the temperature of said heatin means.

23. Apparatus according to claim 7, including means for varying the amount of heat applied by said heating means to said increments.

References Cited UNITED STATES PATENTS 3,169,156 2/1965 Johnson et al. 72342 3,460,364 8/1969 Kralowetz 72- -342 2,952,294 9/1960 Beverly et al. 72364 3,060,564 10/1962 Corral 72342 3,315,513 4/1967 Ellenburg 72392 3,193,270 7/1965 Dewez et al. 72342 CHARLES W. LANHAM, Primary Examiner M. J. KEENAN, Assistant Examiner U.S. Cl. X.R. 

